This invention relates to fiber optic cables and, more particularly to fiber optic cables that include a plurality of optical fibers in one or more buffer tubes.
Optical fibers are very small diameter glass strands which are capable of transmitting an optical signal over great distances, at high speeds, and with relatively low signal loss as compared to standard wire or cable (including wire cable) networks. Many applications of optical fibers require the individual fibers to be placed into groupings, such as in fiber optic cables.
Fiber optic cables are widely used in communications systems. One type of fiber optic cable, referred to as a unitube fiber optic cable, includes an outer jacket surrounding a tube, which contains one or more optical fibers.
Cold temperatures can adversely affect fiber optic cable, since the temperature coefficient of expansion (TCE), also called the coefficient of thermal expansion (CTE), is quite large for dielectric materials, typically plastics or other non-glass materials, versus the optical fiber. When the cable is exposed to cold temperatures, the cable structural elements contract more than the fiber. In unitube fiber optic cables, there are commonly three approaches to obtaining cold temperature performance. One is to have adequate free space in the tube, the second is to have stiffening rods built into the cable and the third is a combination of free space and stiffening rods.
Typically in a unitube cable there is some free space in the tube, which encases the fiber to allow the fiber to assume a serpentine (sinusoidal) type shape as the cable structure contracts. If this effect is too large it can cause optical attenuation that results in an unusable cable. The tube can be made large enough to accommodate this effect. However, this increases the overall diameter of the cable.
Manufacturers typically use a combination of this free space with either metallic or dielectric strength members whose TCE is very close to that of optical fiber and that have a high modulus ( greater than 50 Gpa is typical). These strength members restrict the contraction of the composite cable minimizing the amount of free space required in the unitube. This approach also increases the diameter of the cable, as the profile of the strength members is typically round and the outer jacket must have adequate thickness to prevent the strength members from separating from the jacket when the cable is exposed to bends.
In addition, the use of these strength members can create additional issues. When only two strength members are used, they are typically oriented 180 degrees apart and are located either in the outer jacket of the cable or at the inside wall of the outer jacket. This creates a preferred bend orientation, since the two strength members, with their high modulus, will cause the cable to twist until the strength members are on the neutral axis of bend.
There is a need for a fiber optic cable that can withstand low temperatures and avoids the disadvantages of prior designs.
Fiber optic cables constructed in accordance with this invention include an outer jacket, a first core tube positioned within the outer jacket, and a first plurality of optical fibers positioned within the first core tube, wherein the cross-sectional area of the first plurality of optical fibers is less than 60 percent of the cross-sectional inside area of the first core tube and wherein the length of each optical fiber in the first plurality of optical fibers is between 1.0 and 1.001 times the length of the first core tube.
Strength members can be positioned between the outer jacket and the first core tube. The core tube and outer jacket can be made of a material selected from the group consisting of: polyvinyl chloride, polyvinylidene fluoride homopolymer, and polyvinylidene fluoride copolymer.
In another embodiment, a second core tube can be positioned within the outer jacket, and a second plurality of optical fibers can be positioned within the second core tube, wherein the cross-sectional area of the second plurality of optical fibers is less than 60 percent of the cross-sectional inside area of the second core tube and wherein the length of each optical fiber in the second plurality of optical fibers is between 1.0 and 1.001 times the length of the second core tube. The second core tube can be aligned substantially parallel to the first core tube and the outer jacket can define a tearable web section in between the first and second core tubes.
In another embodiment, a plurality of core tubes can be positioned within the outer jacket, and optical fibers can be positioned within each of the plurality of core tubes, wherein the total cross-sectional area of the optical fibers in each of the core tubes is less than 60 percent of the cross-sectional inside area of the core tube in which those optical fibers are located, and wherein the length of each optical fiber is between 1.0 and 1.001 times the length of the core tube in which each optical fiber is located. The plurality of core tubes can be helically wound with respect to each other. Electrical conductors can be positioned within the outer jacket and the electrical conductors can be helically wound with respect to each other and the core tubes.